Altostratus clouds are categorized by meteorologists as mid-level formations. These clouds are defined by their expansive, sheet-like structure, which typically blankets the entire sky. Their presence often suggests a transition in the current weather pattern. Altostratus clouds cover vast geographical areas without showing much distinct texture or shape.
Appearance and Defining Features
Altostratus clouds appear as a uniform, featureless sheet that is typically gray or a pale bluish-gray color. The cloud layer covers the entire visible sky from horizon to horizon. They are generally thick enough to create an overcast condition, but they are not as dark or ominous as the rain-bearing nimbostratus clouds.
The most distinctive feature of an altostratus deck is its effect on the sun or moon. When these clouds are present, the sun’s position may be visible as a bright, circular patch, but its edges are diffused and watery. This phenomenon is often described as the “watery sun” because the light is scattered and muted. The sun or moon is never sharp enough to cast shadows on the ground, and unlike cirrostratus clouds, altostratus clouds do not produce a halo effect.
The texture of the cloud layer is mostly smooth and uniform, though sometimes it may show a slightly fibrous or striated appearance. If the cloud is thick enough, the sun’s outline will be completely obscured, and the sky will simply look uniformly gray. This overall lack of distinctive features makes the altostratus cloud one of the more visually subtle of the ten main cloud genera.
Altitude and Make-up
Altostratus clouds are classified as mid-level clouds, which is indicated by the prefix “alto,” derived from the Latin word altum, meaning “high.” The term stratus comes from the Latin word for “layer” or “spread out,” aptly describing their flat, layered form. In temperate regions, these clouds generally form with their bases between 2,000 and 7,000 meters (6,500 to 23,000 feet) above the surface.
The composition of altostratus clouds is a mixture depending on the temperature within the cloud layer. The lower portions are often made up of supercooled water droplets, which remain liquid despite temperatures being below freezing. The upper parts consist of tiny ice crystals, which form due to the colder temperatures at higher altitudes. This dual composition allows the cloud to produce its characteristic diffused appearance.
Associated Weather Patterns
The presence of altostratus clouds is a strong indicator of a significant change in the weather, often signaling the approach of a warm front or an occluded front. These clouds form as a large mass of warm, moist air is slowly lifted over a cooler air mass. The broad, gentle uplift causes the moisture to condense into this expansive mid-level cloud layer.
Altostratus clouds frequently precede nimbostratus clouds, and observing them suggests that continuous, widespread precipitation is likely within the next 12 to 24 hours. While they can produce very light rain or snow, this precipitation often evaporates before reaching the ground, a phenomenon known as virga. If the cloud layer thickens and lowers, and the precipitation begins to reach the ground consistently, the cloud has then transitioned into a nimbostratus.